A U.S. envoy called on North Korea Tuesday to release three detained American citizens, saying that the issue poses a "serious impediment" to better ties with Washington.

Glyn Davies, the top U.S. nuclear envoy, was in Seoul as part of his three-nation Asian tour that already took him to China. The tour, which also includes a stop in Tokyo, is for discussions on ways to reopen the long-stalled six-party talks on denuclearizing North Korea.

During his one-day stay in Beijing, Davies made a similar call for North Korea to set free the three Americans, including Kenneth Bae, on humanitarian grounds. Outside analysts believe that the North is holding the Americans as leverage to force the United States to open bilateral talks with it.

"It remains a significant serious impediment to the betterment of our relations with North Korea," Davies told a group of reporters in Seoul. "It remains for us a source of real concerns that North Korea not only won't release them, but it won't talk to us about how we might go forward to secure their release."

The envoy said that Washington has sought to have "conversations" with North Korea over the issue and expressed its intention to send senior American diplomats to resolve it, but the North has rejected such offers.

"It really does put the lie to North Korea's insistence (or) contention that they truly seek better relationship with the U.S.

If so, let our people go," he said.

In Beijing, Davies openly denounced North Korea for using the three U.S. detainees as political "pawns" but said behind-the-scenes diplomacy is under way to seek their release.

Accompanied by Sydney Seiler, the new special envoy for the six-party nuclear talks, and Allison Hooker, director for Korea at the National Security Council, Davies flew to Seoul from Beijing earlier in the day and later met with his South Korean counterpart, Hwang Joon-kook.

He was scheduled to fly to Tokyo on Wednesday. His main agenda during the trip is how to reopen the six-party talks that have been dormant since late 2008, according to South Korean officials. The six-party forum involves, the two Koreas, the U.S., China, Japan and Russia.

North Korea, which walked away from the negotiating table, is now asking for an "unconditional" resumption of the talks, but Seoul and Washington insist that Pyongyang first take concrete steps toward denuclearization.

Last week, the United Nations' International Atomic Energy Agency (IAEA) slammed North Korea for advancing its nuclear capabilities, such as the restart of a nuclear reactor that is believed to have the ability to produce nuclear bomb material.

Before holding the talks with Davies, Hwang said that Seoul and Washington need to "take stock of the current situations and consider the next steps that they are going to take in concert."

Davies told reporters later that Pyongyang has the responsibility to take action to fulfill its earlier pledge to give up its nuclear weapons and to have "meaningful discussions" with parties in the six-party talks.

"That's where the onus has to be and that's where it's important that the international community continues to keep the pressure on," he said. "It is important that North Korea begins now to address these concerns (related to the nuke issue)."

When it comes to criticism for the effectiveness of the six-party talks, Davies said that the forum is "the right architecture" in addressing concerns about the North's nuke threats.

Davies also said in Beijing that North Korea is accused of "even more directly rejecting" calls by its neighbors and the international community to honor its earlier pledges for denuclearization.

He added that Washington and Beijing have "firmly" agreed to the importance of denuclearizing North Korea. (Yonhap)